The Black Cats prop up the Premier League table as things stand and travel to face a free-scoring Everton side unbeaten in eight matches since before Christmas.
However, Koeman stressed the importance of preventing confidence from turning into complacency and insists that his side will be punished if they don't play at their best.
"It's always difficult. They fight against relegation. They also had a break. In the last couple of days I watched the Sunderland-Southampton game. The final score was 4-0 but the first 45 minutes Sunderland was the better team," he told reporters.
"We need to prepare ourselves. If you're not 100%, if you take it too easy, if you are sloppy they will punish you for that. We are full of confidence but we have to show that in a good level on the pitch. The team and the people are happy. But in a good run you have to be prepared for what can happen. We know we are strong but it's football and we need to keep the momentum and that starts this weekend."
The match will also see a first return to Goodison Park for Darron Gibson and Bryan Oviedo following their January moves to Sunderland, and Koeman believes that they will be afforded a warm welcome back to Merseyside.
"It's always special for the players. They went to Sunderland to get more game time, that was a good choice by both players. They will get a respectful welcome. If they play they will do everything to win the game for Sunderland. We are friends after the game and before the game but during the 95 minutes everyone is involved in his own team," he added.
Victory for Everton could see them close the gap on the Champions League places to just six points.